When capturing photographic shots of an illuminated scene by an electronic camera, specular reflections of the lighting in heavily reflective objects to be photographed may occur in the image area or region of interest. Especially round objects, such as bottles, couch-work of a car and exhaust pipes of cars or motor-cycles may cause small, very bright spots, by reflection of studio lamps. The brightness (expressed in lumen/(steradian.m.sup.2)) of such spots may be up to eight times higher than the brightest surface, which diffuses the light of the lighting. A specular reflecting object may be seen as an object that reflects more light than a white object. It has been experienced that about one third of the camera shots are affected by specular reflection. Specular reflection in planar objects may be avoided by slightly moving or rotating the object. For curved or round objects however, such as bottles, pipes, spherical or cylindrical objects, it may be very difficult or impossible to move or rotate the object such that specular reflections are totally eliminated. Alternatively, specular reflections may be diminished by application of an anti-reflective coating to the specular objects, but this requires various skills from the photographer and is rather time consuming. Placing shades to prevent glare from shiny surfaces may be too cumbersome.
Specular reflections are especially a problem in digital photography, where a digital camera is used for acquisition of an image from a scene. Even if lighting conditions are selected such that lights are positioned to give equal illumination across the entire scene, lighting the scene may result in specular glare from the lights. It is hard to avoid that a spherical or cylindrical object has a glare portion causing specular reflection highlights by illumination from the various light sources. A highlight is a bright area in an image. Highlights can either be caused by a diffuse reflection or a specular reflection. By specular reflection, the lighting ratio may be severely disturbed. The lighting ratio is the ratio of the illumination in the darkest shadows--at least, those that are intended to represent visible details--to the brightest highlights. Usually, the equivalent of 41/2 f-stops difference in illumination is visible in image reproductions. f-Stops or f-numbers are commonly known as numbers that appear on the barrel of a lens and represent the different sizes of aperture available.
The aperture is the circular opening or diaphragm within the lens that determines the amount of light allowed to pass through to reach the photosensitive film or elements. A small aperture corresponds to a large f-stop number, for greater depth of field. The depth of field is the distance of acceptably sharp focus extending both in front of and behind the point of true focus. It varies, depending on the aperture selected, the focal length of the lens and the focal distance. A smaller aperture, shorter focal length or increased focal distance increases the depth of field. Experienced photographers have a good idea of the required aperture for a given distance of the scene and a distance variation within the scene.
Moving the aperture ring up one stop (for example from f/4 to f/5.6) makes the aperture smaller and halves the amount of light passing through the lens. Reducing the diameter of the aperture by a factor of 1.4=.sqroot.2, reduces the area of it by a factor of 2. Such a reduction may be compensated by doubling the exposure time, or by doubling the luminous intensity of the lighting. Dividing the focal length of the lens by the effective diameter of the aperture yields the f-number. For example, a 110 mm lens and an aperture diameter of 10 mm would equal f/11.
Specular reflections may give problems in automatic adjustment of exposure time of the camera. In a digital camera, exposure time may be controlled by software. In CCD cameras, exposure time basically corresponds to the reading time of the CCD. Based on the brightness of the specular reflections, the exposure time may be chosen too short, which results in an obvious under-exposure of the useful image area. Moreover, if the exposure time is too short, then a low signal to noise ratio may arise and blur the digital image. By increasing the exposure time or charge integration time, conventional CCD sensors may give blooming problems. Setting the exposure time correctly to cope with specular reflections, will avoid blooming of CCDs but will render the main scene too dark. Setting the exposure time correctly for the "main scene" causes the main scene to appear good, but may cause blooming effects in CCD arrays. As a consequence, highlights are "burnt out", i.e. no detail is visible in the highlight regions.
Dynamic range is a measure of how wide a range of subject luminance (e.g. brightness) a sensor can accurately reproduce. Just about any digital camera can capture a wider range of brightness values than the printed page can reproduce. Therefore, the software or hardware must compress the full range of scene brightness into a range that printing devices can reproduce. This capability is called tone compression. Different devices may use different input-to-output curves to accomplish it. As a general purpose, tone compression avoids losing either the highlights or the shadows, yet still maintaining reasonable contrast in the midtones. The resulting tone curve may be S-shaped, flattening at the extremes, with a steeper slope in the midtone area.
The photographer may make several exposures, varying the lens aperture, the integration time or for linear CCD arrays the line time, brightness of the lighting conditions and so on.
In the past, several attempts have been made to adjust tone curves, merely by avoiding the influence of possibly present specular reflections. MegaVision, Inc., located in Santa Barbara, Calif., introduced ToneBall, a tool for adjusting a camera's tone curve. It comprises three wooden balls--one white, one black and one grey--glued together. Unlike flat grey-scale control strips, which can catch specular glare from the lights, ToneBall is round; thus the photographer can always find a glare-free portion of the sphere on which to measure the reference colours. According to the ToneBall method, specular reflections are merely avoided rather than spotted and corrected for, as proposed in the description of the invention herein below.
EP-A-0 335 419 describes a method and apparatus for establishing highlight and shadow densities on the basis of a cumulative histogram of densities in an original. A reference density X.sub.HR --associated with a prescribed reference statistic value Y.sub.HR --is compared with a prescribed threshold density X.sub.HS, to select one out of two possible methods for establishing a gradation correction curve. The method described in this prior art document merely finds out whether the distribution of highlight densities is "normal" or "abnormal". A high occurrence or frequency of highlight densities would classify the image as normal and select a usual gradation correction curve. A low occurrence of highlight densities would initiate exceptional processing for an abnormal original by shifting the gradation curve so that the gradation correction curve does not include useless part at its highlight end. Specular reflections however give a remarkable occurrence of highlight densities and though require exceptional processing according to the method described herein below.
In Photoshop (a trademark of Adobe, Inc.) an "auto-level function" is available. Application of this function considers "specular reflections" as "highlights in the main scene" and therefore may compress the dynamic range of the "main scene" such that too much density variations--though originally present in the raw image--are lost by the gradation correction. The "main scene" is that portion of the image that contains the most relevant information for the photographer. This may include text or logos on partly specular objects, etc. Moreover, tone correction may be different for different colour channels and may therefore degrade the colour balance of the digital image. If the brightest spot has no true neutral white colour, then colour casting may occur.